{"id":"F2006B09136","name":"AD/DHC-6/48 - Introduction of Beta Range Test Switch","slug":"ad-dhc-6-48-introduction-of-beta-range-test-switch","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":115691,"registerId":"commonwealth-F2006B09136-current","compilationNumber":null,"startDate":"2026-04-03","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/DHC-6/48 - Introduction of Beta Range Test Switch","content":"---\nmeta-content-style-type: text/css\nmeta-content-type: application/xhtml+xml; charset=utf-8\nmeta-generator: Aspose.Words for .NET 22.10.0\n---\n\n?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"no\"?>\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n \n\n![](image.001.png)  DHC-6 (Twin Otter) Series Aeroplanes\n\n \n\nAD/DHC-6/48 Introduction of Beta Range Test Switch 2/80\n\nApplicability: DHC-6 Aircraft S/No. 1 thru 450.\n\n \n\nDocument: 1. De Havilland Aircraft of Canada Ltd., SB No. 6/309, Rev. C; or\n\n \n\n2. Transport Canada Airworthiness Directive CF-79-24 De Havilland Canada. Compliance:              Within next 500 hours time in service after 31 March 1980.\n\nBackground: Investigations of two accidents overseas where inadvertent reverse thrust was suspected, drew attention to the need to ensure that serviceability of the beta back-up system could always be checked before flight. Incorporation of the beta range test switch ensures that such a check can be accomplished.\n\n \n","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The directive applies to DHC-6 aircraft S/No. 1–450 and requires incorporation of a beta range test switch to permit preflight checking of the beta back-up system, which matches the stated purpose in the Background. The published applicability and the remedial measure are aligned with the directive's stated intent; the text does not indicate an expansion or narrowing beyond that stated scope (Applicability; Document; Background)."},"complexity_factors":["Narrow technical requirement limited to a defined set of airframes (Applicability)","Clear, single mechanical remedy (install beta range test switch) with two referenced compliance documents (Document)","Fixed and short compliance window (500 hours after 31 March 1980) (Compliance)","Safety justification based on two overseas accident investigations (Background) rather than an extensive regulatory framework","No cost or procedural detail in the text — requires reference to external documents for implementation (Document)"],"plain_english_summary":"### What this directive does\n\nThis airworthiness directive requires De Havilland DHC-6 (Twin Otter) aeroplanes with serial numbers 1 through 450 to have a \"beta range test switch\" incorporated so that the beta back-up system can be checked before flight. The directive directs compliance by using either the manufacturer service bulletin (De Havilland SB No. 6/309, Rev. C) or Transport Canada Airworthiness Directive CF-79-24 (Document). It sets a deadline of within the next 500 hours time in service after 31 March 1980 for operators to comply (Applicability; Document; Compliance).\n\n### Who issues and who must comply\n\nThe directive is issued under the Commonwealth Civil Aviation Regulations and appears in the Civil Aviation Safety Authority schedule of Airworthiness Directives (header). Aircraft owners/operators of DHC-6 S/No. 1–450 are the parties required to obtain and apply the modification and the associated preflight check capability (Applicability; Document).\n\n### Why it was introduced (stated reason)\n\nThe background expressly states investigators examined two overseas accidents in which inadvertent reverse thrust was suspected. The stated purpose of installing the beta range test switch is to ensure the serviceability of the beta back-up system can always be checked before flight (Background). That is the mechanism the directive uses to address the stated safety concern.\n\n### How it works mechanically\n\n- The operator must incorporate the beta range test switch in affected aircraft according to one of the two referenced technical documents (Document).  \n- Once incorporated, the switch provides a means to check the beta back-up system during preflight checks (Background).  \n- The directive gives a finite compliance window: within 500 hours time in service after 31 March 1980 (Compliance).\n\n### Who pays, who decides, and what changes in behaviour\n\n- Who pays: The source does not state a funding source. Practically, aircraft owners/operators will bear the direct costs of parts, labour and any associated downtime to install the switch (Document; Compliance).  \n- Who decides: The Civil Aviation Safety Authority enforces the airworthiness directive under the Civil Aviation Regulations; the operator or maintenance organisation decides how to schedule and execute the modification, following one of the referenced technical documents (header; Document).  \n- Behavioural change required: Operators must plan and carry out the installation within the stated compliance window and then use the switch to perform preflight checks of the beta back-up system (Document; Compliance; Background).\n\n### Practical trade-offs, compliance burden and risks\n\n- Compliance burden: The directive applies to a defined set of aircraft (S/No. 1–450), so the compliance cost is concentrated on those owners/operators (Applicability). The source does not provide cost estimates or procedural detail beyond the citation of the two documents (Document).  \n- Scheduling and availability: Because compliance is time-limited (500 hours after 31 March 1980), operators must incorporate the modification into upcoming maintenance cycles; this can impose short-term downtime or reallocation of maintenance resources (Compliance).  \n- Implementation risk: The directive's safety mechanism is the ability to check the beta back-up system before flight. If the switch is not fitted correctly, or if the check is not performed or is ineffective, the intended safety benefit will not be realised (Background; Document).  \n- Procedural discretion and alternatives: The directive permits compliance by following either the manufacturer service bulletin or the Transport Canada AD, which provides two documented paths to achieve the required modification (Document). The authority still rests with the Civil Aviation Safety Authority to require the AD (header).\n\n### Effects on markets and operators\n\n- The directive imposes a technical retrofit on a defined, older fleet segment. The direct economic impact (parts, labour, downtime) falls on the aircraft owners/operators; there is no pricing, competition or ownership change described in the source text.  \n- Because the requirement is narrow and targeted to specific serial-numbered aircraft, effects on wider competition or industry structure are likely limited in scope; costs are concentrated on the affected aircraft rather than diffused across an entire market (Applicability).\n\nSources cited in text: header (authority), Applicability, Document, Compliance, Background."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, time-bound safety directive with a single purpose. It has not expanded beyond its original intent of mandating a specific hardware modification to address a known safety risk."},"complexity_factors":["Single, specific technical requirement (install one switch)","Narrow applicability (only DHC-6 aircraft, serial numbers 1-450)","Clear compliance deadline (500 hours after 31 March 1980)","Only two alternative reference documents for compliance method","No nested definitions, cross-references to other legislation, or conditional exceptions","Background explanation provided in plain language"],"plain_english_summary":"This is an **Airworthiness Directive** — a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for specific aircraft.\n\n**What it does:**\nRequires owners and operators of certain De Havilland DHC-6 \"Twin Otter\" aircraft (serial numbers 1 through 450) to install a **beta range test switch**. This is a safety device that lets pilots test whether the aircraft's backup reverse-thrust system is working properly before takeoff.\n\n**Why it matters:**\nTwo overseas accidents suggested that the aircraft could accidentally go into reverse thrust during flight — a dangerous malfunction. The test switch ensures pilots can verify this critical safety system is functional before every flight.\n\n**Who must comply:**\n- Owners and operators of affected DHC-6 Twin Otter aircraft\n- Must be done within 500 flight hours after 31 March 1980\n\n**How to comply:**\nFollow either:\n- De Havilland Service Bulletin No. 6/309 (Revision C), **or**\n- Transport Canada Airworthiness Directive CF-79-24\n\nThis is a **retrospective safety modification** — it applies to existing aircraft, not just new ones."}},"importantCases":[],"_links":{"self":"/api/acts/ad-dhc-6-48-introduction-of-beta-range-test-switch","history":"/api/acts/ad-dhc-6-48-introduction-of-beta-range-test-switch/history","analysis":"/api/acts/ad-dhc-6-48-introduction-of-beta-range-test-switch/analysis","conflicts":"/api/acts/ad-dhc-6-48-introduction-of-beta-range-test-switch/conflicts","importantCases":"/api/acts/ad-dhc-6-48-introduction-of-beta-range-test-switch/important-cases","documents":"/api/acts/ad-dhc-6-48-introduction-of-beta-range-test-switch/documents"}}